ANO Art 21(4)(a) says:
a) For the purposes of this Part of this Order ... a licence granted either under the law of a Contracting State other than the United Kingdom ..., shall..., be deemed to be a licence rendered valid under this Order but does not entitle the holder:
(i) to act as a member of the flight crew of any aircraft flying for the purpose of public transport or aerial work or on any flight in respect of which he receives remuneration for his services as a member of the flight crew; or
(ii) in the case of a pilot's licence, to act as pilot of any aircraft flying in controlled airspace in circumstances requiring compliance with the Instrument Flight Rules or to give any instruction in flying.
That suggests to me that a non-JAA licence/rating doesn't permit you to give instruction in a G-reg and certainly doesn't allow you to be paid for it.